Add an Admin API to forget all rooms with 0 local members

**Description:**

Since [`forget_rooms_on_leave`](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#forget_rooms_on_leave) defaults to false, enabling [`forgotten_room_retention_period`](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#forgotten_room_retention_period) on your server may leave you with a bunch of old un-forgotten rooms on your server. An Admin API to forget all local rooms with 0 local members would solve this and allow you to easily clean up old unused rooms after enabling `forget_rooms_on_leave` and/or `forgotten_room_retention_period`.

The alternative would be to manually look over all the rooms on your server and manually deleting all rooms with 0 local members. But this can be tedious if you have lots of rooms or users.